1. Field of the Invention
The present invention relates to a method and a device for controlling an analog system providing an output signal with a control signal.
2. Discussion of the Related Art
An application of the present invention relates to the control of a signal amplifier of a portable phone transmission system.
FIG. 1 shows in the form of a block diagram a conventional architecture of a portable phone transmission system 10.
Transmission system 10 comprises an amplifier 12 (AMPLI) receiving an analog signal S comprising the information to be transmitted corresponding, for example, to a modulated voltage. Amplifier 12 transmits an amplified analog signal S′ to an antenna 14 (ANTEN).
Transmit system 10 generally receives a digital reference PREF generally transmitted to the portable phone by a distant terminal and for example corresponding to a determined power value at which signal S′ is to be transmitted.
Amplifier 12 is driven by an analog control signal VC which corresponds to the conversion by a digital-to-analog converter 16 (DAC) of a digital control signal NC provided by a calculator 18 (F). Calculator 18 receives signal PREF and has access to a memory (not shown) in which is stored a control table indicating theoretical values of the control signal for the power values aimed at.
In practice, the power of signal S′ may differ from the theoretical power according to certain parameters such as the operating temperature of amplifier 12, the portable phone supply voltage, or other external parameters. It is thus necessary to correct the theoretical control signal. This is currently performed by a closed-loop control of the power of signal S′ provided by amplifier 12.
A sensor 20 provides based on signal S′ an analog detection signal VD to the non-inverting input of a comparator 22 (COMP). Signal VD may correspond to a voltage equal to the peak value of signal S′ or to the average value of signal S′. A generator 24 (GEN) provides a reference signal VREF, generally a constant voltage, to the inverting input of comparator 22 which transmits to an analog-to-digital converter 26 (ADC) a modified detection analog signal V′D corresponding to the difference between signal VD and reference signal VREF. Converter 26 converts signal V′D into a digital detection signal N′D transmitted to calculator 18. Calculator 18 also has access to a memory (not shown) where is stored a detection table indicating for given values of signal N′D the corresponding values of the power of signal S′. Signal NC is provided by calculator 18, which corrects the above-mentioned theoretical value by the indication provided by the detection table.
In FIG. 2, curves 30 and 32 represent, for given operating conditions of the amplifier, voltages VC and VD according to power POUT of signal S′ expressed in dBm. Power POUT thus is equal to 10 logP, where P is the power of signal S′ expressed in milliwatts. More specifically, curve 30 represents the real relation, for the given operating conditions, between voltage VC and power POUT of signal S′ really provided by amplifier 12 upon reception of signal VC.
The regulation of the power of output signal S′ is performed as follows. Based on power reference PREF, calculator 18 determines a first value, or theoretical value, of digital control signal NC based on the control table. The value of signal VC is that for which the amplifier should theoretically provide a signal S′ of power PREF. Based on signal N′D, calculator 18 determines the power really provided by amplifier 12 and accordingly increases or decreases the value of signal NC.
As appears in FIG. 2, signal VD can vary between two positive values VMIN and VMAX. Signal VREF is generally taken to be equal to VMIN. Signal V′D can thus vary between 0 and VMAX-VMIN. The maximum value of V′D (VMAX-VMIN) is thus smaller than the maximum value of VD (VMAX). Signal V′D can thus be coded by a smaller number of bits than signal VD and converter 26 can be simplified.
The number of bits of converter 26 is also a function of the accuracy with which the power of signal S′ is desired to be controlled. In the present example, the power amplitude is 74 dBm. If the accuracy desired for the power control of output signal S′ is 0.2 dB, calculator 18 must be able to provide 370 distinct values of NC. Considering that the relation between V′D and power POUT is substantially linear and that to each value of V′D between 0 and VMAX-VMIN corresponds a power value, this amounts to saying that converter 26 must be able to code 370 different values for signal N′D. A coding over 9 bits enables coding 512 different values of voltage V′D. The amplitude of voltage V′D being of 1260 millivolts, the accuracy obtained for the coding of voltage V′D is approximately 2.5 millivolts. To obtain a coding of same accuracy for signal VD which has an amplitude of 2187 mV, a coding over 10 bits will be necessary.
As mobile telephony standards vary, the accuracy required for the power control of the output signal tends to increase. It will then be necessary to increase the number of bits used by analog-to-digital converter 26 to code a greater number of different values of voltage V′D.
This results in an increase of the cost of analog-to-digital converter 26 which is hardly compatible with the cost constraints specific to mobile telephony. Further, according to the required accuracy, the technological limits of the analog-to-digital converters used in mobile telephony and which enable coding to approximately 14 bits may be reached.